The present invention relates generally to printed circuit boards, and deals more particularly with a capacitor formed within a printed circuit board.
Printed circuit boards are widely used today to mount and interconnect electrical components such as discrete resistors, discrete capacitors, transistors, digital circuits, etc. It is common for the printed circuit board to contain many layers. Typically most of the components are mounted on the surface. Some of the conductors used to interconnect the components are also printed on the surface. The inner layers are primarily used to interconnect the components through other conductors printed on these inner layers and conductive vias passing through the outer and inner layers. For complicated circuits, the surface area must be carefully allocated to fit the many requisite components. Also, it is desirable to position some of the capacitors near to other, associated components to minimize path length and thereby minimize parasitic inductance.
It was also previously known to form a discrete capacitor from a bottom aluminum electrode, a next layer of tantalum (Ta), a next layer of Ta.sub.2 O.sub.5 serving as a dielectric, and a top electrode layer. This capacitor was mounted on the surface of a substrate, and two conductive vias passed through the substrate to connect to the two electrodes. This capacitor was not part of a printed circuit board, but instead was a surface component on a substrate.
To conserve surface area of a printed circuit board, it may be desirable to form some of the capacitors within the printed circuit board. This will reduce demands on the surface area and also permit a capacitor to be located near to an associated component, if there is no space available on the surface near to this component.
Various techniques were previously known to form a capacitor within a printed circuit board. See for example, U.S. Pat. Nos. 5,079,069 and 5,161,086. While theses techniques may be effective in their respective printed circuit boards and for their respective applications, further improvements are desirable to provide a high amount of capacitance per amount of inner layer area utilized, provide a fabrication process with acceptable cost and complexity and provide a fabrication process which is compatible with the requisite printed circuit board materials, such as epoxy, polyimide or Teflon polymers and polymer impregnated glass cloth or fiber laminate materials and the like. Epoxy resins typically have glass transition temperatures from about 120.degree. C. to about 190.degree. C. and thermal decomposition temperatures from about 300.degree. C. to about 375.degree. C. Epoxy resins will withstand short duration exposures above the glass transition temperature, but will not withstand temperature excursions above the thermal decomposition temperature. For example, the fabrication process required for the capacitor cannot require so much heat or harsh chemicals as to degrade the printed circuit board materials.